Silicoaluminophosphates (SAPO) are taught in U.S. Pat. No. 4,440,871, for example. SAPO materials are both microporous and crystalline and have a three-dimensional crystal framework of PO2+, AlO2− and SiO2 tetrahedral units and, exclusive of any alkali metals or other cation which may optionally be present, an as-synthesized empirical chemical composition on an anhydrous basis of:mR:(SixAlyRz)O2 wherein “R” represents at least one organic templating agent present in the intracrystalline pore system; “m” represents the moles of “R” present per mole of (SixAlyRz)O2O2 and has a value of from 0 to 0.3, the maximum value in each case depending upon the molecular dimensions of the templating agent and the available void volume of the pore system of the particular SAPO species involved; “x”, “y”, and “z” represent the mole fractions of silicon, aluminum, and phosphorus, respectively, present as tetrahedral oxides. The minimum value for each “x”, “y”, and “z” is 0.01 and preferably 0.02. The maximum value for “x” is 0.98; for “y” is 0.60; and for “z” is 0.52.
Typically, the silicoaluminophosphate molecular sieves are synthesized by hydrothermally crystallizing a hydrous gel made from substantially homogeneous aqueous reaction mixture containing reactive sources of aluminum, phosphorus, silicon and the other element(s), if any, required in the molecular sieve. The reaction mixture also preferably contains an organic templating, i.e., structure-directing, agent, preferably a compound of an element of Group VA of the Periodic Table, and/or optionally an alkali or other metal. The reaction mixture is generally placed in a sealed pressure vessel, preferably with an inactive metallic surface or alternatively, lined with an inert plastic material such as polytetrafluoroethylene and heated, preferably under autogenous pressure, at a temperature between 50° C. and 250° C., and preferably between 100° C. and 200° C., until crystals of the non-zeolitic molecular sieve product are obtained. Usually this is for a period of from several hours to several weeks. Effective crystallization times from about 2 hours to about 30 days are generally employed. The molecular sieve is recovered by any convenient method, for example, centrifugation or filtration.
It is disclosed in U.S. Pat. No. 4,440,871 that while not essential to the synthesis of SAPO compositions, it has generally been found that stirring or other moderate agitation of the reaction mixture and/or seeding the reaction mixture with seed crystals of either the SAPO species to be produced or a topologically similar aluminophosphate or aluminosilicate composition, facilitates the crystallization procedure. These silicoaluminophosphates exhibit several physical and chemical properties which are characteristic of aluminosilicate zeolites and aluminophosphates.
U.S. Pat. No. 4,943,424 describes a SAPO molecular sieve designated SM-3. It is characterized to distinguish it from all other silicoaluminophosphate forms as being a silicoaluminophosphate having a phosphorus, silicon, and aluminum concentration at the molecular sieve surface that is different than the phosphorus, silicon, and aluminum concentration in the bulk of the molecular sieve, and having the essential X-ray diffraction pattern of SAPO-11.
None of the U.S. patents mentioned above discloses or teaches how to make the crystalline silicoaluminophosphate molecular sieve of this invention.